The present invention relates to power calibration of an optical pickup unit (OPU) with respect to a target command during a mass production phase of an optical disc drive, and more particularly, to a method for deriving precise control over laser power of an OPU, and to an associated automatic power calibration (APC) circuit.
Regarding the control over an OPU of an optical disc drive in the related art, a conventional APC circuit can be utilized for controlling the laser power of a laser diode (LD) during a normal operation of the optical disc drive, e.g. a reading/writing operation. When the conventional APC circuit reaches a steady state during the normal operation mentioned above, the laser power corresponds to a target command sent to the conventional APC circuit. It is a goal for the conventional APC circuit to control the laser power to be a specific power value corresponding to the target command, in order that the laser power varies in accordance with the target command. Sometimes, the goal mentioned above appears to be too idealized to achieve, the reason for which is described as follows.
A conventional method for deriving the relationship between the laser power and the target command typically comprises measuring the laser power by utilizing a power meter, and collecting data sets of the laser power and the target command. However, the cost of the power meter is high, and the corresponding tooling and labor costs of a power calibration station for implementing this method are also required. Additionally, another issue such as the differences between respective power calibration stations may arise.
According to the related art, an OPU vendor may design a front-end photo diode (PD) in an OPU, and the manufacturers (e.g. an optical disc drive manufacturer) uses the front-end PD as a replacement for the power meter. The measurement result from the front-end PD is outputted through a front-end PD output (FPDO), and can be referred to as the FPDO value. Some examples of curves of a relationship between the laser power of the OPU and the FPDO value are illustrated in FIG. 1. As shown in FIG. 1, the curve passing through the origin corresponds to an ideal case, and the other two curves correspond to a real case with a positive offset and another real case with a negative offset.
As the OPU vendor typically provides a few data points for stating the relationship between the laser power and the FPDO value, interpolation operations are required for deriving the laser power corresponding to other data points on a predicted curve passing through the few data points mentioned above. As a result, the whole process of deriving a precise relationship between the laser power and the target command is slowed down due to the interpolation operations.
In addition, when trying to derive the relationship as mentioned with the laser power having a duty cycle such as 50% (e.g. the 50% duty cycle write power), it is very hard to accurately measure a direct current (DC) component by utilizing the FPDO value due to various hardware limitations, e.g. analog bandwidth.
Additionally, the gain and the offset of the conventional APC circuit may vary from chip to chip, and the overall gain and the overall offset of the combination of the OPU and the conventional APC circuit may also vary from system to system. Thus, when using the FPDO, the plan of deriving a precise relationship between the laser power and the target command does not work well in practice.